The present invention relates to noise attenuation systems. In particular, the present invention relates to noise attenuation systems for use with gas turbine engines such as aircraft auxiliary power unit (APU) turbine engines.
Large commercial aircraft typically include on-board APU turbine engines, located in the tail sections of the aircraft, to provide electrical power and/or compressed air for systems throughout the aircraft. When an aircraft is on the ground, the primary propulsion engines of the aircraft are shut down, and the APU turbine engine provides the main source of power for a variety of systems, such as the environmental control systems, hydraulic pumps, electrical systems, and main engine starters. The APU turbine engine may also provide power during in-flight operations, such as for electrical and pneumatic systems.
In many gas turbine engine applications, particularly those in which the engine is used in conjunction with a commercial passenger aircraft, there is a widespread demand by the airline industry to maintain noise levels below defined limits. This is particularly important at ground service stations for the aircraft, where ground crew load and unload luggage, fuel and provision the aircraft, and remove waste materials from the aircraft. Under these conditions, the aircraft APU is the turbine engine of interest.
Noise generated during the operation of an APU turbine engine typically includes low frequency noise generated during the combustion process within the turbine engine, and high frequency noise generated by the interaction with inlet air at the compressor portion of the turbine engine. The low frequency noise is typically attenuated with an exhaust silencer placed downstream from the APU exhaust diffuser. This allows the exhaust silencer to attenuate the noise of the combustion gases as the gases exit the exhaust diffusers.
The high frequency noise, however, is typically attenuated with the use of an inlet noise silencer disposed in an air inlet duct, where the air inlet duct is located upstream relative to the APU turbine engine. To provide effective attenuation of high frequency noise, the inlet noise silencer desirably has a narrow passage width that is comparable in size with the wavelengths of the high frequency noise. If the passageway is too large (e.g., greater than about twice the average noise wavelength), the level of noise attenuation decreases.
One common technique to provide small passageways in an air inlet duct involves segregating the duct into separate parallel passages with the use of acoustically lined splitters. Unfortunately, the splitters can collect ice on their leading edges, which may clog the air inlet duct, thereby hindering certification of the APU turbine engine for operation in icing conditions. Furthermore, the use of splitters increases the cost of the inlet noise silencer. As such, there is a need for an inlet noise attenuation system that is effective for attenuating high frequency noise without the use of splitters or other components that may otherwise hinder the use of the APU turbine engine.